Method of inhibiting corrosion in steam generating systems



Unite States Patent I 3,029,126 Patented Apr. 10, 1 962 tihce 3 029 126 METHOD OF INHiBI'fING CORROSION IN STEAM GENERATING SYSTEMS Joseph F. Chittum, Whittier, Califi, assignor to Cali This invention relates to a highly effective method of retarding corrosion of ferrous metals in steam generating systems, particularly in condensate lines thereof.

The usual steam generating systems comprise a boiler in which water is heated to temperatures ordinarily in the range from 250 F. to 500 F. to generate steam, a steam line to carry the steam from the boiler to the point of use, the heat exchangers, turbines, etc., where the steam is condensed, and a condensate line to return condensed spent steam to the boiler drum. Steam generating Systems are ordinarily constructed of iron or steel, which corrodes in normal usage, with the result that maintenance costs are increased and equipment life is shortened. Particularly susceptible to corrosion are the sections of the steam plant wherein the steam is condensed, i.e., in the zone of condensation. One particular form of steam generating plant is the steam system used in stripping gasoline fractions from rich oil separated in a natural gasoline plant. In such systems the condensed steam contains corrosive amounts of carbon dioxide and sometimes oxygen.

In accordance with the present invention, effective inhibition of corrosion of the steam and condensate lines of steam generating systems can be obtained When nitroalkanes of no more than four carbon atoms are introduced into the feed water in small amounts, usually in the range of 0.001 to 0.2 percent by weight of the total water (liquid and vapor) in the steam generating system. Stated in terms of steam rate, the niroalkane is introduced at a rate of A1 to 2 quarts per day per 10,000 pounds per hour of steam. Further, it has been found that the nitroalkanes are most effective when the content of salt (namely, alkali metal chloride) in the condensed water is below about 500 p.p.m., preferably below 100 p.p.m., calculated on the basis of all salts being present as sodiurn chloride. In a test with the salt content above 500 p.p.m., thev addition of a nitroalkane, such as Z-nit-ropropane, to the boiler water was found to increase surprisingly the corrosivity rather than decrease it. Therefore, the introduction of nitroalkane into the water to inhibit corrosion of ferrous metal in the condensing zone of a steam system is advantageously accompanied by maintenance of the concentration of alkali metal chlorides in the condensed water below the above-mentioned concentration. By carrying out the treatment in such manner, corrosion of the ferrous metal parts of the steam system is greatly inhibited.

Illustrative of the nitroalkanes of no more than four carbon atoms that may be employed in accordance with the present invention, are the following: nitromethane, nitroethane. l-nitropropane, 2-nitropropane, l-nitrobutame, and 2-nitrobutane. These compounds are liquids and may be used as such or dissolved in an inert carrier, such as water or a hydrocarbon solvent, e.g., kerosene, methanol, and the like.

Data indicating the unusual effectiveness of the method of the present invention in inhibiting corrosion in steam generating plants are summarized in the following Table l. The nitroalkanes were introduced continuously at the indicated rates into a steam generating plant. running at a steam rate of 33,000 pounds per hour at a temperature of 300 F. and at a pressure varying throughout the system from 60 to 600 p.s.i.g. The chloride content of the boiler water was maintained below 20 p.p.m., calcu- I lated as sodium chloride. Iron coupons were suspended in the condensate line wherein the corrosion is most difiicult to inhibit. Corrosion rates are expressed in mils per year and were calculated from the-weight loss of the coupons during the tests. A suitable corrosion measuring device is described at page of the January 21, 1957,

issue of the Oil and Gas Journal.

TABLE 1 Time After Test Injection Start of Corrosion Series Inhibitor Rate Test Series Rate Number (QtSJday) (hrs) (mils/yr.)

1 z-uitropro- 2 2 Palm 1. 8-85 3. 2 -do 2. 85-95 2. -150 3. -170 3. 290-420 4 5 1 4.3 -230 Egon?l (fror)n 260-285 6.

- 30 ours 4 Nitrome- 2.8 285-290 Rapidly mane decreasing.

Inspections of the steam generating plant showed that corrosion was also being inhibited in the less sensitive portions thereof.

The relationship of injection rate of nitroalkanes to the corrosion rate are shown in Table 3, the measurements being made as described above, except that the injection rates have been converted to the basis of 10,000 pounds per hour of steam.

TABLE 2 Inhibitor Injec- Corrosion Rate Inhibitor tion Rate (Qts. (Mils./year) per day) The above data clearly illustrate the marked effective ness of the nitroalkanes in reducing corrosion in steam generating plants and that the lower molecular Weight nitroalkane, namely nitromethane, was more eiiective on an equal unit 'volume rate basis.

The data in following Table 3 illustrate that the effectiveness of the nitroalkanes is diminished as the chloride content of the total water in the steam generating system isincreased to 100 p.p.m. and to 500 p.p.m., calculated on a sodium chloride basis:

claim:

1. The method of inhibiting corrosion due to steam condensing in the presence of carbon dioxide and oxygen in the condensing zone of a steam generating system where water is cycled through a boiler converting liquid water to steam, the condensing zone with steam condensing and normally causing corrosion at the elevated temperature and a condensate line returning condensed spent steam to the boiler, which method comprises maintaining a concentration of alkali metal chlorides in thetotal water, liquid and vapor, in said system below 500 p.pl rn., calcu lated on the basis of all chlorides being present as sodium chloride, and introducing into the Water in said system a quantity of mononitroallgane having no more than four carbon atoms, said quantity being suflicient to give the water a concentration of said nitroalkane in the range of 0.001 to about 0.2 percent by weight.

2. The method of inhibiting corrosion due to steam condensing in the presence of carbon dioxide and oxygen in the condensing zone of a steam generating system where water is cycled through a boiler converting liquid water to steam, the condensing zone with steam condensing and normally causing corrosion at the elevated temperature and a condensate line returning condensed spent steam to the boiler, which method comprises maintaining the concentration of alkali metal chlorides in the total water, liquid and vapor, in said system below 100 p.p.rn., calculated on the basis of all chlorides being present as sodium chloride, and introducing into the water in said system a mononitroalkane having no more than four carbon atoms at a rate in the range of to 2 quarts per day of said nitroalkane per 10,000 pounds of steam per hour.

3. The method of claim 2 wherein said nitroalkane is 2-nitrop1 opane.

4. The method of claim 2 wherein said nitroalkane is nitrornethane.

References (Jited in the tile of this patent UNITED STATES PATENTS 2,185,238 Whaley Jan. 2, 1940 2,411,593 Routson Nov. 26, 1946 2,532,407 Iohansson g Dec. 5, 1950 2,562,571 Partridge July 31, 1951 2,582,138 Lane 3am. 8, 1952 2,639,971 Broyles May 26, 1953 2,771,417 Ryznar et a1, Nov. 20, 1956 2,872,281 Kahler et al Feb. 3, 1959 FOREIGN PATENTS 282,743 Switzerland Sept. 1, 1952 OTHER REFERENCES U-hlig; Corrosion Handbook, John Wiley and Sons, Inc, New York, 1948, pp. 959, 970. 

1. THE METHOD OF INHIBITING CORROSION DUE TO STEAM CONDENSING IN THE PRESENCE OF CARBON DIOXIDE AND OXYGEN IN THE CONDENSING ZONE OF A STEAM GENERATING SYSTEM WHERE WATER IS CYCLED THROUGH A BOILER CONVERTING LIQUID WATER TO STEAM, THE CONDENSING ZONE WITH STEAM CONDENSING AND NORMALLY CAUSING CORROSION AT THE ELEVATED TEMPERATURE AND A CONDENSATE LINE RETURNING CONDENSED SPENT STEAM TO THE BOILER WHICH METHOD COMPRISES MAINTAINING A CONCENTRATION OF ALKALI METAL CHLORIDES IN THE TOTAL WATER, LIQUID AND VAPOR IN SAID SYSTEM BELOW 500 P.P.M., CALCULATED ON THE BASIS OF ALL CHLORIDES BEING PRESENT AS SODIUM CHLORIDE AND INTRODUCING INTO THE WATER IN SAID SYSTEM A QUANTITY OF MONOITROALKANE HAVING NO MORE THAN FOUR CARBON ATOMS, SAID QUANTITY BEING SUFFICIENT TO GIVE THE WATER A CONCENTRATION OF SAID NITROALKANE IN THE RANGE OF 0.0001 TO ABOUT 0.2 PERCENT BY WEIGHT. 